I need to add income to a Schedule C on an already amended 2018 tax return (amend the amendment). I do not see the Schedule C on the downloaded 2018 tax file. Any ideas?

Correct, the starting point for the second 2018 Form 1040-X should be the first 2018 Form 1040-X. 

To resolve the missing schedule C, first revisit your tax history to see if there is a different download for the amended return. It would be best to start with the first amendment ,tax file. In that case, opening an amendment from the amended .tax file should resolve the missing Schedule C issue.

If it is not seen on the tax history, the first amended .tax file is not available on your computer or the first Form 1040-X that was filed for 2018 in your files cannot be found, contact the IRS.

• Go to the IRS Get Transcripts online tool. You will want Record of Account for the type of transcript for all the details.
• You may want to also go to this link to verify your Amended Return Status with the IRS, that they did receive the first amendment.

Once you have a copy of the first Form 1040-X, use this to fill in your second Form 1040-X by following these steps:

• Search for amend and answer the questions in the 2018 program to create Form 1040-X.
  • Stop entering any information here for now and go to the Forms mode:
    • From your 2018 CD/Download, click Forms in the top right corner.
    • Form 1040-X should be listed already. Scroll to find it or click on the Open Form file box icon on the top left side and enter 1040-X in the search.
      • Select Form 1040-X: Amended Tax Return and click Open Form. 
      • Scroll down to Column A and enter Colum C from the first Form 1040-X.
  • Return to the Step-by-step and continue by entering the changes to your Schedule C

See: Reporting Self-Employment Business Income and Deductions